In recent years, there is proposed an active stabilizer system as described in JP-A-2002-518245, namely, a stabilizer system capable of changing roll restraining force to be exerted or generated by a stabilizer bar in accordance with turning conditions of a vehicle, etc. Such active stabilizer system is installed actually on some vehicles. The stabilizer system includes a stabilizer apparatus constituted by including a stabilizer bar and an actuator. By controlling the operation of the actuator, the rigidity of the stabilizer bar is changed, thereby actively changing the roll restraining force to be exerted by the stabilizer bar.